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Hazardous Area Temperature Class Table (T1 to T6)
Temperature class table giving the maximum surface temperature allowed for Ex equipment in each T class and typical gases that need it.
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Temperature classes
| Class | Max surface °C | NEC subclasses (°C) | Example gases |
|---|---|---|---|
| T1 | 450 | Hydrogen, methane, propane, ammonia | |
| T2 | 300 | T2A 280, T2B 260, T2C 230, T2D 215 | Acetylene, ethylene, butane, ethanol |
| T3 | 200 | T3A 180, T3B 165, T3C 160 | Petrol, diesel, kerosene, hexane, hydrogen sulphide |
| T4 | 135 | T4A 120 | Acetaldehyde, diethyl ether |
| T5 | 100 | Few common gases | |
| T6 | 85 | Carbon disulphide |
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About this Table
Temperature class table giving the maximum surface temperature allowed for Ex equipment in each T class and typical gases that need it.
Basis: IEC 60079 0 temperature classification and NEC Article 500 subdivisions. A gas needs a T class whose maximum surface temperature is below its autoignition temperature.
How to use
- Find the lowest autoignition temperature of the gases in the area.
- Pick a class with a maximum surface temperature below it.
- A T6 device is safe in a T1 to T5 area; not the reverse.
Notes
- Surface temperature limits are at the stated ambient range (usually −20 to 40 °C); hotter ambients may need derating.
- Check the gas data in IEC 60079 20 1 for your actual substances.
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Frequently Asked Questions
What is the maximum surface temperature for T4?
135 °C.
Which T class is needed for hydrogen?
T1, because its autoignition temperature is about 560 °C.
Can T3 equipment be used in a T4 area?
No. T3 allows 200 °C, above the 135 °C limit of T4.
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Learn more on the blog
Temperature classes in hazardous areas
A practical guide on instrumentationblog.in that explains the engineering behind this table.
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